1. Field of the Invention
This invention relates to an improvement in the characteristics of nonlinear resistance elements of a metal-insulating film-metal (MIM) structure formed on substrates of a liquid crystal display.
2. Description of the Related Art
In recent years, the number of picture elements of a display using a liquid crystal display panel has been increasing more and more. In a system which uses multiplex driving for a display of a passive matrix structure, as the time division grade becomes high, the contrast or the response speed degrades and if about 200 scanning lines are contained, it becomes difficult to provide a sufficient contrast.
To eliminate such disadvantages, a liquid crystal display panel of an active matrix system which provides a switching element for each picture element is adopted.
The active matrix liquid crystal display panels are roughly classified into the 3-electrode family using thin film transistors such as triodes and the 2-electrode family using nonlinear resistance elements such as diodes. The 2-electrode family is excellent in terms of structure and manufacturing method which are simple and easy.
The diode type, varistor type, MIM type, etc., are developed for the 2-electrode liquid crystal display panels; particularly, the MIM type has features of a simple structure and short manufacturing process.
High density and high definition are also required for liquid crystal display panels, so it is necessary to make the element area small.
For this purpose, ultramicro photo lithography and etching techniques used to manufacture LSIs or VLSIs of semiconductor integrated circuits can be used. Those skilled in the art focus on the use of MIM of simple structure as switching elements for a large area which also enable cost reduction.
Next, the MIM element structure effective for a large area or cost reduction is described in conjunction with the accompanying drawings.
FIG. 1 is a plan view showing the structure of a liquid crystal display using nonlinear resistance elements. FIG. 2 is a sectional view taken on line I-I' of FIG. 1. The liquid crystal display in the related art is described in conjunction with FIGS. 1 and 2.
As shown in FIG. 2, a first electrode 32 is formed on a first substrate 31 and a nonlinear resistance layer 33 is formed on the first electrode 32. Further, a second electrode 34 is formed on the nonlinear resistance layer 33, and the first electrode 32 and the second electrode 34 are opposite to each other via the nonlinear resistance layer 33 to make up a nonlinear resistance element 30. A part of the second electrode 34 also serves as a display electrode 35.
On a second substrate 36, a black matrix 37 (shielding layer) hatched in FIG. 1 is formed to prevent light from leaking from gaps in display electrodes 35 formed on the first substrate 31.
Further, on the second substrate 36, an opposite electrode 39 is formed so as to be opposite to the display electrode 35. The opposite electrode 39 is formed on the black matrix 37 via an insulating film 38 so as not to be in contact with the black matrix 37.
As shown in FIG. 1, the first electrode 32 formed on the first substrate 31 forms an overhanging area to form the nonlinear resistance element 30, and the overhang area overlaps with the second electrode 34 to form the nonlinear resistance element 30.
The first electrode 32 and the display electrode 35 have a given clearance between them.
The display electrode 35 and the opposite electrode 39 are positioned on the opposite sides so as to overlap each other, thereby making up a picture element part of a liquid crystal display panel.
The black matrix 37 is formed so as to project to the formation area of the display electrode 35 for the purpose of preventing light from leaking from the periphery of the display electrode 35.
The liquid crystal display performs predetermined display in response to a transmittance change of liquid crystal in an area in which the black matrix 37 on the display electrode 35 is not formed.
The first and second substrates 31 and 36 are formed each with an orientation film 40 as a processing layer to regularly arrange molecules of liquid crystal 41.
Further, a spacer 42 is provided to make the first and second substrates 31 and 36 opposite to each other at a predetermined interval, and liquid crystal 41 is enclosed between the first and second substrates 31 and 36.
The liquid crystal display, which does not emit light by itself, requires a display lighting section 45 as external light. The display lighting section 45 is disposed on the side of the second substrate 36 forming the black matrix 37.
Since the black matrix 37 is formed on the second substrate 36 opposite to the nonlinear resistance element 30, light is not emitted to the nonlinear resistance element 30.
FIG. 3 is a plan view showing the structure of a liquid crystal display using nonlinear resistance elements 7 having a form different from that of the nonlinear resistance element shown in FIGS. 1 and 2. FIG. 4 is a sectional view taken on line II-II' of FIG. 3. The liquid crystal display is described in conjunction with FIGS. 3 and 4, and parts identical with or similar to those previously described with reference to FIGS. 1 and 2 are denoted by the same reference numerals in FIG. 3 and 4 and will therefore not be discussed again.
A first electrode 32 is formed on a first substrate 31 and a nonlinear resistance layer 33 is formed on the first electrode 32. Further, a second electrode 34a is formed so as to overlap on the nonlinear resistance layer 33, and a nonlinear resistance element 30a is formed by using the side wall of the first electrode 32. A part of the second electrode 34a also serves as a display electrode 35.
When the nonlinear resistance element 30a is formed by using the side wall of the first electrode 32, the overlap portion of the first and second electrodes 32 and 34a via the nonlinear resistance layer 33, which is an insulating layer, reduces and the area of the nonlinear resistance element 30 can be made small. In addition, the parasitic capacitance of the element can be reduced.
A numeral 45 is a display lighting section similar to that in FIG. 2. Since it is disposed on the side of a second substrate 36, light is shielded by a black matrix 37, and light is not emitted to the nonlinear resistance element 30a.
Some nonlinear resistance elements show an asymmetrical change depending on the polarity of applied voltage. Characteristic examples of a nonlinear resistance element having the asymmetrical characteristic are given in conjunction with the accompanying drawings.
FIG. 5 is a graph showing voltage-current characteristics of nonlinear resistance elements each using tantalum (Ta) as a first electrode, tantalum oxide (Ta.sub.2 O.sub.5) as a nonlinear resistance layer, and indium tin oxide (ITO), a transparent conducting film, as a second electrode.
In the graph of FIG. 5, curve L denotes the initial characteristic of the nonlinear resistance elements shown in FIGS. 2 and 4; curves M1 and M2 denote the characteristics after the nonlinear resistance elements shown in FIGS. 2 and 4 are driven respectively.
For the curves M1 and M2 showing the characteristics after the nonlinear resistance elements in FIGS. 2 and 4 are driven, when a positive voltage is applied to the first electrode of the nonlinear resistance element, the value of current that can flow into the nonlinear resistance element at the same voltage lowers greatly compared with that for the curve L indicating the initial characteristic.
For the curves M1 and M2 showing the characteristics after the nonlinear resistance elements in FIGS. 2 and 4 are driven, when a negative voltage is applied to the first electrode of the nonlinear resistance element, the value of current that can flow into the nonlinear resistance element at the same voltage lowers slightly compared with that for the curve L indicating the initial characteristic.
When the nonlinear resistance element is formed using the side wall of the first electrode as shown in FIGS. 3 and 4, the film quality of the nonlinear resistance layer of the side wall part is bad compared with the top of the first electrode, thus the characteristic change amount is greater than that of the nonlinear resistance element in FIG. 2.
Assume that P1 is the difference between the curve L indicating the initial characteristic when a positive voltage is applied to the first electrode (Ta) and the curve M1 indicating the characteristic after the nonlinear resistance element in FIG. 2 is driven and that P2 is the difference between the curve L and the curve M2 indicating the characteristic after the nonlinear resistance element in FIG. 4 is driven. Likewise, assume that Q1 is the difference between the curve L indicating the initial characteristic when a negative voltage is applied to the first electrode (Ta) and the curve M1 indicating the characteristic after the nonlinear resistance element in FIG. 2 is driven and that Q2 is the difference between the curves L and M2.
As seen in FIG. 5, the differences P1 and P2 when a positive voltage is applied to the first electrode are much greater than the differences Q1 and Q2 when a negative voltage is applied to the first electrode.
Further, FIG. 6 shows changes of the differences described with the graph in FIG. 5 according to the driving time.
In the graph in FIG. 6, curves R1 and R2 denote changes of the differences P1 and P2, respectively, when a positive voltage is applied to the first electrode according to the driving time; the current change values rise rapidly with the driving time.
In contrast, curves S1 and S2 denote changes of the differences Q1 and Q2, respectively, when a negative voltage is applied to the first electrode according to the driving time; the amount of current changes only a little although the driving time increases.
The changes are denoted by the difference between the curves R1 and S1, U1, and between the curves R2 and S2, U2; the differences U1 and U2 increase rapidly as the driving time is prolonged.
The differences U1 and U2 change depending on the current amount flowing into the nonlinear resistance elements, the environment in which the nonlinear resistance elements are driven, and the history of the nonlinear resistance elements in addition to the driving time described with FIG. 6.
Thus, it is extremely difficult to compensate for the changes of the differences U1 and U2.
Since the differences U1 and U2 occur, the voltage applied to a liquid crystal picture element, when a positive voltage is applied to the first electrode of the nonlinear resistance element shown in FIG. 2, 4, differs from that when a negative voltage is applied to it. Thus, DC voltage is applied to liquid crystal and ions in the liquid crystal are biased. If a fixed pattern is displayed for a long time, a burn phenomenon of an image, a residual image phenomenon, occurs in which the pattern is seen as a residual image if the screen is changed. In addition, image flickering occurs and the display quality of the liquid crystal display degrades remarkably.
Although the area occupied by the nonlinear resistance element 30a shown in FIG. 4 can be made small by using the nonlinear resistance layer formed on the side wall of the first electrode 32, symmetry of characteristic change is bad, causing the display quality of the liquid crystal display to degrade.